In recent years, a mobile radio apparatus, such as a mobile phone or the like, has quickly been developed. The mobile phone is used in close proximity to a human body in most of the cases. When an antenna of the mobile phone closes to a human body, an impedance of the antenna will change. When the impedance of the antenna changes, impedance matching between the antenna and a transmission/reception circuit will be shift from its matching state. When the impedance matching shifts, properties of the mobile phone will be deteriorated. For that reason, there is proposed a technology for automatically matching the impedance between the antenna and the transmission/reception circuit.
For example, there is proposed in Patent Document 1 a radio apparatus which is provided with a variable load section within an impedance matching circuit. According to the conventional radio apparatus described in Patent Document 1, when the impedance shifts from the matching state resulting from closing to a human body, the impedance between a radio unit and an antenna will be matched by means of adjusting a load of a matching circuit.
FIG. 67 is a block diagram illustrating a configuration of a conventional radio apparatus 900 described in Patent Document 1. In FIG. 67, the conventional radio apparatus 900 is provided with an impedance matching circuit 904 for adjusting an impedance between a radio unit 907 and an antenna 901 with a variable capacitance diode. Electric power detection circuits 906 and 902 generate signals C and D for indicating electric powers of signals at the preceding stage and the subsequent stage of the impedance matching circuit 904, respectively. A comparator 905 compares the voltage signals C and D. An applied voltage control circuit 903 generates a voltage signal V for controlling a capacitance of a variable capacitance diode based on a comparison result E so that a difference between the voltage signals C and D may become small to thereby supply it to the variable capacitance diode. According to the voltage signal V, the impedance matching circuit 904 adjusts the impedance. As a result of this, the conventional radio apparatus can adjust the impedance between the radio unit 907 and the antenna 901.
In addition, there is disclosed in Patent Document 2 a method of matching an impedance between an antenna and a power amplification section.
According to the conventional method described in Patent Document 2, when one of a plurality of channel frequencies is specified, the impedance between the power amplification section and the antenna is adjusted in a specified channel. According to this conventional method, when a channel frequency is set, by rotating a variable element in a forward or reverse direction to a setting tuning position of each channel frequency by means of a drive motor using a manual signal, a preset adjusting operation for changing a load as an antenna dummy resistance, and a managing operation for correcting a preset value with a manual signal in order to match it to an actual antenna impedance are provided. As a result of this, the impedance between the antenna and the power amplification section will be matched in the specified channel.
Moreover, there is disclosed in Patent Document 3 an impedance matching device capable of automatically matching an impedance between an antenna and a transmitting section when the environment around the antenna changes suddenly.
The conventional impedance matching device disclosed in Patent Document 3 is provided with a standing-wave ratio detecting section for measuring a plurality of voltages at respective points over a transmission path from the transmitting section to the antenna to detect a standing-wave ratio, an impedance calculating section for calculating a present impedance on an antenna side seen from a side of the transmission path, based on the aforementioned standing wave over the aforementioned transmission path, a setting match table for storing, as a list, setting values of a predetermined element for matching which eliminates a difference between an impedance on a side of the transmitting section and the present impedance calculated as above, a variable matching section provided for impedance adjustment in an input terminal of the antenna which receives a transmission power from the aforementioned transmission path, and an operation control section for controlling a matching element of the aforementioned variable matching section to be a predetermined value based on the present impedance and the aforementioned setting match table. As described above, the conventional impedance matching device disclosed in Patent Document 3, while storing the setting value of the predetermined element for matching which eliminates the difference between the impedance on the side of the transmitting section and the present impedance on the side of the antenna, adjusts a variable matching circuit so as to provide this setting value, and matches the impedance between the transmitting section and the antenna.
Furthermore, there is disclosed in Patent Document 4 an antenna tuner for selectively coupling, with an antenna and a transmitter-receiver, a plurality of capacitors and a plurality of inductors which serve as a load of the antenna and are selectively connected between the antenna and the transmitter-receiver, based on whether a load resistance is larger than a characteristic impedance of the antenna or small. As a result of this, an antenna system can be tuned.
[Patent Document 1] Japanese Laid-open Patent Publication No. 11-251928
[Patent Document 2] Japanese Laid-open Patent Publication No. 60-80323
[Patent Document 3] Japanese Laid-open Patent Publication No. 8-97733
[Patent Document 4] Japanese Laid-open Patent Publication No. 57-101435